makes identical cells
Control of Cell Division and Cell Death
• Human cells appear to be programmed to
undergo only so many cell divisions
– About 50 in cell cultures
• During fetal development, many cells
are programmed to die, apoptosis
Fingers and toes form from these
paddlelike hands and feet
Apoptosis happens if cells of the human myeloid cell line are
deprived of growth factors. SEM 10,000.
Only cancer cells and stem cells can divide endlessly
Controlling the Cell Cycle
• The eukaryotic cell cycle is controlled by feedback at three checkpoints
– 1. Cell growth is assessed at the G1 checkpoint
– 2. DNA replication is assessed at the G2 checkpoint
– 3. Mitosis is assessed at the M checkpoint
•
Cancer is characterized by cells that have lost the ability to control their division
see Figure 8.9
G0 is an extended
rest period
What is Cancer?
• Cancer is unrestrained cell growth and division
• Most cancers result from mutations in growth-regulating genes
• There are two general classes of these genes
– 1. Proto-oncogenes
• Encode proteins that simulate cell division
• If mutated, they become oncogenes
– 2. Tumor-suppressor genes
• Encode proteins that inhibit cell division
–Can undergo
metastasis
–Leave the
tumor and
spread
throughout the
body
• Cancer can be caused by chemicals, radiation or viruses
• Rous Sarcoma Virus (RSV) chicken cancer from cell-free extracts
- Peyton Rous 1911 Nobel Prize 1966
- Oncogenes - cancer causing genes
- Protooncogenes- normal genes which stimulate cell growth
- growth factors, receptors,
signal transduction proteins, etc
Cancer and Control of the Cell Cycle
• The p53 gene plays a key role in the G1 checkpoint
of cell division
• The p53 protein (the gene’s product), monitors the
integrity of DNA
– If DNA is damaged, the protein halts cell division
and stimulates repair enzymes
• If the p53 gene is mutated
– Cancerous cells repeatedly divide
– No stopping at the G1 checkpoint
New molecular therapies for cancer
Receiving the
signal to divide
Stopping
tumor growth
Stepping
on the gas
Passing the
signal via a
relay switch
Amplifying
the signal
Releasing
the “brake”
Checking that
everything is ready
Prokaryotes Have a Simple Cell Cycle
• Prokaryotic cells divide asexually
– These cells possess a single
circular chromosome, containing
about 1000 genes
– The chromosome is replicated
– The cell then divides into two cells,
a process called binary fission
– 17 million daughter cells in 8-hours
Eukaryotes Have a Complex Cell Cycle
• Cell division in eukaryotes is more complex than in
prokaryotes because
– 1. Eukaryotic contain far more DNA
– 2. Eukaryotic DNA is packaged differently
• It is in linear chromosomes compacted with proteins
• Some eukaryotes also
make exact copies of
themselves via asexual
reproduction
amoeba
The Cell Cycle
Interphase
Figure 8.5
– G1 phase
• Primary
growth phase
– S phase
• DNA
replication
– G2 phase
• Microtubule
synthesis
– M phase
(MITOSIS)
• Chromosomes
pull apart
– C phase
(CYTOKINESIS)
• Cytoplasm
divides
Chromosomes
•The DNA helix is wrapped
around positively-charged
proteins, called histones
~200 nucleotides of DNA +
histones= nucleosome
•During interphase, the DNA is
coiled into chromatin but not
visible chromosomes
•A chromosome is a complex of
one molecule of DNA (~ 40%)
and proteins (~ 60%)
•A typical human chromosome contains about 1000 genes
^Figure 8.2
~ 140 million nucleotides in its DNA, ~5 cm in stretched length
Homologous Chromosomes
• Diploid cells have two
SIMILAR* chromosomes
Figure 8.4
called homologous
chromosomes or
homologs
• Humans have 46
chromosomes or 23
pairs of homologs
(similar chromosomes, one
from this person’s Mom and
one from their Dad)
* Important
The chromosomes can be
organized by size into a karyotype
Sister Chromatids
• Replicated chromosomes
consist of two IDENTICAL*
sister chromatids held
together at the centromere
• DNA replication occurs in
S phase (Sister)
• Homologs are SIMILAR*
chromosomes
* Important
see Tables 8.1 & 8.2 for terms
• Before a cell starts
dividing, the
chromosomes
duplicate
• Because the
strands of the
double helix are
complementary
A+T
C+G
an exact copy of
the DNA can be
made
• An emergent
property
• Chromosomes
duplication (DNA
replication or
Synthesis)
produces
IDENTICAL
sister chromatids
• When the cell divides,
the sister chromatids separate
• Each of the two daughter cells has a
complete and identical set of chromosomes
MITOSIS makes identical diploid (2n) cells
MEIOSIS makes haploid (1 n) gametes
Eukaryotic cells divide in
one of two ways
– Mitosis
• Occurs in
somatic (nonreproductive)
cells
– Meiosis
• Occurs in germ
(reproductive)
cells
• Results in the
production of
gametes
Haploid gametes (n = 23)
Egg cell
Sperm cell
MEIOSIS
FERTILIZATION
Diploid
zygote
(2n = 46)
Multicellular
diploid adults
(2n = 46)
Mitosis and
development
Mitotic Cell Cycle
– M phase MITOSIS
• Chromosomes pull apart
–
–
–
–
Prophase
Metaphase
Anaphase
Telophase
– C phase CYTOKENESIS
• Cytoplasm divides
INTERPHASE
Figure 8.5
Chromosomes are
decondensed during
interphase
chromatin
– G1 phase
• Primary Growth phase
– S phase
• DNA Synthesis
– G2 phase
• Growth & Microtubule
synthesis
Prophase
• Chromosomes coil
• Nuclear membrane
breaks down
• Spindle fibers form
see Figure 8.7
see Figure 8.7
Metaphase
• Chromosomes
line up on the
midline
• Spindle fibers
attach to
centromeres
Anaphase
• Centromeres divide
• Spindle fibers shorten
• Sister chromatids
separate and move to
opposite poles
see Figure 8.7
Telophase
• Cell elongates
• Nuclear membrane reforms
•
• Chromosomes uncoil
• Spindle disappears
see Figure 8.7
Cytokinesis
• Division of the cytoplasm
• Cleave furrow forms at
equator of the cell
• Constriction tightens by
contraction of filaments
• Cell is divided into two
identical cells
see Figure 8.7
• Cytokinesis
see Figure 8.8
• Animal cells
• Plant cells
– Cleavage furrow forms,
– Cell plate forms,
pinching the cell in two
dividing the cell
in two
skin
Dead
cells
- Asexual
reproduction,
development,
growth and cell
replacement
are mitotic
divisions
Epidermis,
the outer
layer of the
skin
Dividing
cells
Dermis